Field of the Invention
The present invention relates to an image forming apparatus forming an image and more specifically to an image forming apparatus including an electrification unit electrifying a surface of an image bearing member such as a photosensitive drum.
Description of the Related Art
Conventionally, an electro-photographic image forming apparatus is widely applied as a copier, a printer, a facsimile machine, and a multi-function printer having a plurality of functions of those apparatuses. Such image forming apparatus adopts a process of electrifying a surface of a photosensitive drum, i.e., an image bearing member, by an electrification roller, of exposing the electrified surface by a laser scanner to form an electrostatic latent image, of applying toner to the electrostatic latent image by a developing unit to form a toner image, and of transferring the toner image onto a sheet to visualize the image.
In order to continuously form favorable images by such image forming apparatus, it is required to replace components thereof in accordance with a service life set for each individual unit and member. For instance, one of factors shortening a service life of an electrification roller, i.e., an electrification member, is an electrification failure of the electrification roller caused by toner external additive depositing non-homogeneously on a surface of the electrification roller. Therefore, as a method for estimating an amount of the external additive depositing on the electrification roller (referred to an ‘additive deposition amount’ hereinafter), a method of detecting that an integrated value of an electrification voltage applied time and an integrated value of a number of rotations of the electrification roller have reached predetermined reference values is developed as disclosed in Japanese Patent Application Laid-open No. Hei. 09-211931, for example. In the image forming apparatus adopting this method, a control portion of the image forming apparatus determines whether or not those values of the electrification roller have reached the reference values and if the control portion determines that those values have reached the reference values, the control portion alarms a user of the image forming apparatus that a time for replacing the electrification roller has come.
However, because the image forming apparatus disclosed in Japanese Patent Application Laid-open No. Hei. 09-211931 estimates the external additive deposition amount on the electrification roller based on the integrated value of the electrification voltage applied time and the integrated value of the number of rotations of the electrification roller, the additive deposition amount varies considerably and may be considerably different from an actual external additive deposition amount. That is, because the amount of the external additive depositing on the electrification roller per unit time varies depending on an image ratio and a usage environment, there is a case when the integrated value of the electrification voltage applied time and the integrated value of the number of rotations of the electrification roller do not correctly reflect the deposition amount depending on an actual use condition.